With Thermal Dissipation Up, Will Cooler Heads (Up) Thinking Prevail?

Greater processing power and component density is driving demand for effective cooling for AdvancedTCA, MicroTCA, CompactPCI Serial and VME systems.

The performance of processor chips has steadily increased and the power density in systems has, as a result, risen exponentially in recent years. For example, in 2002, the AdvancedTCA specification defined 200 watts of heat dissipation for front boards and 15 watts for rear boards. At that time, the ATCA specification was expected to accommodate future increases, but today many boards in use are generating 300 watts per front board and 50 watts per rear board. The latest generation of ATCA boards are dissipating upwards of 400 watts per front board and 50 watts on rear boards, and the next generation of ATCA boards for the telecommunications market features a larger form factor and may produce up to 2 kW per board.

Figure 1. Heat simulations highlight airflow and hot spots.

While not as extreme, this trend can also be seen in other PICMG and VITA specifications. MicroTCA systems, while based on a relatively small form factor, have also increased thermal dissipation by as much as 50 watts for the single modules and 80 watts for double modules. Only a few years ago VME and CompactPCI boards dissipated 20 to 30 watts of heat, but today the figure is around 40 to 50 watts. Today’s VPX specification allows for systems to generate up to 450 watts or more of heat dissipation per slot.

Figure 2. Slot balancing: Air is blocked or restricted to some slots and directed to others.

Airflow Optimization

In the past, air optimization wasn’t as critical as it is today; it wasn’t uncommon to see a system cooling with airflow from bottom to top, with the system above it drawing in the warm air from the system below. Using higher-power fans compensated for poor design, inefficient installation or leaks. Unfortunately, the latest chassis already employ the highest-power fans on the market, so bumping up fan power is no longer an option. The only option to increase cooling is to optimize airflow and maximize efficiency.

Considering the importance of cooling, airflow optimization is a primary consideration for Design Engineers from the earliest stages of system development. (Figure1).

Board design has a direct effect on airflow since heat sinks or densely populated boards create resistance. Cold air will naturally follow the path of least resistance; thus, when combining high resistance and low resistance boards in a chassis, special consideration must be taken to optimize airflow.

While not as extreme, this trend can also be seen in other PICMG and VITA specifications. MicroTCA systems, while based on a relatively small form factor, have also increased thermal dissipation by as much as 50 watts for the single modules and 80 watts for double modules. Only a few years ago VME and CompactPCI boards dissipated 20 to 30 watts of heat, but today the figure is around 40 to 50 watts. Today’s VPX specification allows for systems to generate up to 450 watts or more of heat dissipation per slot.

Figure 3. In a Push Configuration, fans are located in the bottom area close to the air entry and push the air through the system.

Figure 4. In a Pull Configuration, fans or radial blowers are located in the top area next to the exhaust to pull the air through the system.

To better control airflow, air baffles cover empty slots, ensuring that air is directed where it is needed. For systems in which the board configuration does not change, a “slot balancing” element (Figure 2) can be used. This slot balancing element is simply a custom grille fitted under the card cage that redirects air to cards with greater resistance or heat dissipation and away from cards with lower resistance or less thermal dissipation.

Figure 5. In a Push-Pull Configuration, fans are located close to the air entry and close to the air exhaust.

Cooling Specifications

The AdvancedTCA specification allows for a maximum ambient temperature of +40 °C. In emergencies, such as air conditioner failure, ATCA systems can withstand ambient temperatures up to +55 °C for short periods of time (max. 96 hours). For electronic components, however, this “short period” may still be too long as board components also have operating temperature ranges that must be adhered to. When designing cooling systems, it is therefore necessary to assume an ambient temperature of +55 °C. With a +55 °C ambient temperature, the maximum ΔT per slot is 10 K; the temperature may rise by only 10 °C between the air inlet to the air outlet.

Cooling Techniques

Natural convection is the simplest and least expensive method of removing heat; however, it is only practical when the ambient temperature is significantly lower than the required internal temperature of the system. At a room temperature of 20 °C or 25 °C, convection can be used to remove 10 W per slot.

The most common method for convection cooling is forced air, which is sufficient for the majority of situations, and employed in the latest high-performance ATCA systems. With forced air convection, natural convection is augmented with fans or blowers. Here again, the ambient temperature must be significantly lower than the specified system internal temperature. Typically, the temperature in the slot or chassis will be 10 °Kelvin higher than ambient. Some individual components on the boards, such as processors, may be considerably warmer. In some cases, fan trays can be activated when the air resistance of the incorporated components reaches a predefined threshold.

Forced cooling techniques can be divided into three basic approaches: push cooling, pull cooling or a combination of push and pull (Figure 3).

For push cooling, the fans are positioned below the boards and the air is pushed over hot components. Conversely, for pull cooling the fans are mounted above the boards; they pull air up through the card cage. Both the push and pull methods have different strengths and weaknesses when considering operating life, air distribution and air pressure.

High operating temperatures are detrimental both to the mechanical condition of the fans and to the electronics they are meant to keep cool. The fans in the push configuration are situated in a cold airstream and therefore not subjected to high working temperatures, so they tend to have a longer operational life. In the pull configuration, the fans are located at the top of the enclosure within a hot airstream, frequently resulting in a shorter fan life.

Pull systems often use radial blowers to draw the air up from below and blow it horizontally out the rear of the chassis. Radial blowers typically eliminate the need for any redirection of the air, resulting in greater airflow, better air distribution and higher static pressure within the chassis.

If air quality is a concern, the push configuration generates positive pressure in the system, which helps to keep dust and other contaminants out of the chassis. The downside of positive pressure is leakage—some air will inevitably escape through small openings for connectors or gaps between EM gasket elements. In the pull configuration, negative pressure is created in the card cage, which can result in dust being drawn into the system.

It is important to note that the fans themselves also consume power and generate heat. In a push system, heat generated by the fans pre-warms the air reaching the boards whereas in a pull system the heat from the fans simply enters the exhaust airflow.

The latest ATCA systems featuring 450W per slot cooling use push-pull designs, which provide optimum cooling with the greatest levels of air flow, air pressure and most even air distribution. With double the number of fans, the downside of the push-pull configuration is cost and increased fan noise.

Current fan technology restricts greater than 450W per slot cooling in a 55 °C ambient environment. Provided that fan technology does not improve, alternative-cooling methods must be considered.

Air Flow and Thermal Analysis

When evaluating airflow measurements provided by manufacturers, users should always consider the conditions under which the tests were conducted. Test results are not comparable unless the environmental conditions, test equipment, procedures and metrics are consistent. To resolve this issue, the Communications Platforms Trade Association (CP-TA) has developed a standardized measurement protocol, which has been integrated into the PICMG 3.0 specification. The CP-TA method defines measurement cards to be used and the PCB width, slot impedance and airspeed for four locations. Using this protocol, cooling performance for the individual slots can be quantified at each of the four zones and results compared for different configurations.

There is no officially standardized measurement procedure for other bus technologies. For this reason, some manufacturers have defined their own reference boards for MicroTCA with both typical air resistance and the highest air resistance values of PCBs currently on the market and based on the CP-TA method for ATCA. Appropriate boards are also developed for CompactPCI, CompactPCI Serial and VME.
These manufacturers actively test the latest cooling concepts and designs using the latest in measurement and simulation technology. For example, a company’s thermal lab may include a wind tunnel, climate chamber (temperature and humidity) and CP-TA test equipment. On request, manufacturers can also perform testing on customer systems, using their boards, to measure and verify performance.

Hybrid Cooling—Air and Liquid

When convection cooling alone is not sufficient, other alternatives must be considered. One solution is to lower the ambient temperature. By lowering the ambient temperature to +25 °C, the maximum ΔT rises by 30 K from 10 K to 40 K, which makes it possible to cool four times as much heat. The ambient temperature can be adjusted by adding an air/water heat exchanger (AWHE) in the cabinet, with redundancy if high system availability is required. An AWHE from the Schroff LHX series (Figure 4) may remove as much as 1 to 1.5 kW per board given a suitable flow temperature.

Figure 6. Schroff Varistar LHX 12 with side-to-side cooling.

Solutions with AWHEs at a system level rather than a cabinet level are also conceivable, as is the cooling of hotspots directly on the boards. However, it is important to evaluate the costs versus gains of each solution; the greater the complexity, the greater the cost. Energy costs should be considered for overall system design. Often the more complex water-cooled systems will require less energy for ongoing operation. For example, by adding an AWHE, ATCA systems only require a third or a quarter of the fan capacity.

Conduction Cooling

Conduction cooling is the method of using thermally conductive material to transfer heat from the PCB to a cold wall or heat sink. With the clamshell design (Figure 5), electronics are completely encapsulated, and thermal interface materials (TIMs) such as pads, paste, films, adhesives, and even solders are often used to improve the conductivity of the mating surfaces. Conduction cooling is typically used in rugged applications since it provides protection against environmental conditions such as shock, vibration, EMC and contaminants. While conduction cooling alone is limited on a system level, it can be used in conjunction with forced air or liquid cooling at the chassis level for a superior cooling solution. Conduction cooling is also used in environments where airflow is restricted (sealed chassis), or for environments such as in avionics where low-pressure air is not able to remove enough heat.

The demand for greater cooling is expected to continue. In fields such as next-generation AdvancedTCA systems, with dissipations of up to 450W per slot, air cooling has reached its physical limit. In order to continue to meet market demand, fluid cooling will supplement air cooling at the cabinet, system, or board levels. For a wide range of applications, it is still possible to provide adequate ventilation and fan capacity to handle excess heat generated despite growing power density per processor and dissipation loss per unit volume. Of greater importance than previously, however, is the case design, the packing on the boards, the arrangement of the boards in the system and ensuring that empty slots are closed off or optimal airflow is provided.

Christian Ganninger, Dipl.-Ing. (FH) is global product manager for systems products at Pentair Technical Solutions GmbH, Straubenhardt, Germany. Christian began at Schroff, a Pentair brand, in 2005 and has served in many marketing related roles including product manager for backplanes, power supplies, MicroTCA and finally for all systems product lines. In these roles, he has driven product development as well as market and applications analysis. Prior to his time at Schroff, Christian was backplane designer and project manager for a firm that developed 19” systems and backplanes. He studied Electrical Engineering at the University of Applied Science in Karlsruhe, Germany.

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